Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous substances into the atmosphere. Chief among such harmful air pollutants are nitrogen oxides (NO.sub.x). In response to strict enforcement efforts of these laws industrial air polluters have made considerable efforts to reduce the amount of these harmful substances into the air in gaseous effluents from industrial or municipal sources. Successful efforts to reduce the concentration of NO.sub.x in gaseous effluents often involve reacting the NO.sub.x in waste gases with nitrogen-based reducing agents. One commercially used method of reducing NO.sub.x from gas streams involves contacting the NO.sub.x with ammonia or an ammonia precursor, such as urea, in the absence of a catalyst, a technique known as selective non-catalytic reduction (SNCR). The ammonia reduces the NO.sub.x to nitrogen while itself being oxidized to nitrogen and water. Typical SNCR-based processes are disclosed in U.S. Pats. Nos. 5,233,934 and 5,453,258. SNCR processes require very high temperatures, for instance temperatures in the range of about 800 to 1200.degree. C., and even at these temperatures only low conversions of NO.sub.x are achieved. For example, it is not uncommon to attain NO.sub.x reductions in the range of 40 to 50% by SNCR-based processes.
Another technique for removing NO.sub.x from waste gas streams involves contacting the waste gas with ammonia or an ammonia precursor in the presence of a substance which catalyzes the reduction of NO.sub.x to nitrogen, as in SNCR processes. These catalytic reduction processes are referred to as selective catalytic reduction (SCR). SCR processes have a few advantages over SNCR processes. They can be carried out at temperatures significantly lower than the temperatures at which SNCR processes are carried out. For example, they are quite effective at temperatures in the range of about 250 to 600.degree. C. Typical SCR processes are described in detail in U.S. Pats. Nos. 4,119,703, 4,975,256, 5,482,692, 5,589,147, 5,612,010 and 5,743,929. Although SCR processes are much more efficient than SNCR processes in the reduction of NO.sub.x to nitrogen, they have the disadvantages of being more costly than SNCR processes, the catalyst can be poisoned or deactivated and often they do not remove all of the NO.sub.x from the gas stream being treated.
Another disadvantage of both SCR and SNCR processes is that ammonia, which itself is regarded as an environmentally unacceptable pollutant, is often released into the atmosphere in the gaseous effluent from the reactor because the reactions are often conducted in the presence of excess ammonia and/or because of sudden changes in the process that produces the NO.sub.x. Ammonia may also be released because of depletion or masking of the catalyst by contamination over time.
Another known method of removing NO.sub.x from gas streams involves contacting the NO.sub.x with ozone, thereby oxidizing them to higher nitrogen oxides, such as N.sub.2 O.sub.5 and removing the higher oxides from the gas stream by means of aqueous scrubbers.
Specific details of ozone-based NO.sub.x oxidation processes are disclosed in U.S. Pat. Nos. 5,206,002 and 5,316,737, the disclosures of which are incorporated herein by reference. Ozone-based NO.sub.x oxidation processes are quite expensive because of the high cost of producing ozone.
It is also known to oxidize NO to NO.sub.2 and remove the NO.sub.2 from gas streams by contacting a gas stream containing one or both of NO and NO.sub.2 with oxygen, particularly in the presence of an NO.sub.2 -selective adsorbent which contains metal cations. The oxygen oxidizes the NO to NO.sub.2, and the NO.sub.2 is adsorbed from the gas stream by the adsorbent. Typical NO.sub.2 adsorption processes are disclosed in U.S. Pat. Nos. 3,674,429, 4,153,429, 4,160,013, 5,417,950, 5,514,204, 5,670,125 and 5,670,127, the disclosures of which are incorporated herein by reference.
Because of stringent environmental regulations, efforts are continuously made to improve NO.sub.x removal processes to minimize or eliminate emission of NO.sub.x into the atmosphere. This invention provides a process which accomplishes this objective.